MXPA06013444A

MXPA06013444A - Anode for oxygen evolution.

Info

Publication number: MXPA06013444A
Application number: MXPA06013444A
Authority: MX
Inventors: Paolo Rossi
Original assignee: De Nora Elettrodi Spa
Priority date: 2004-05-20
Filing date: 2005-05-19
Publication date: 2007-03-01
Also published as: TWI265214B; CN1957112B; EP1756333B1; AU2005245599B2; RU2006145304A; EP1756333A1; CN1957112A; JP5059605B2; MY142728A; US20080023341A1; AU2005245599A1; KR101201689B1; US8083921B2; KR20070012721A; ZA200609264B; WO2005113861A1; ES2581210T3; ITMI20041006A1; TW200540297A; BRPI0511437A

Abstract

An electrode for high overvoltage oxygen anodic evolution is described comprising a substrate of titanium or other valve metal, a first protective interlayer containing valve metal oxides, a second interlayer containing platinum or other noble metal, and an outer layer comprising tin, copper and antimony oxides. The electrode of the invention may be employed as anode in waste water treatment.

Description

ANODE FOR EVOLUTION OF OXYGEN FIELD OF THE INVENTION The invention relates to an anode for oxygen evolution at high overvoltage in aqueous solutions, for example to destroy organic materials in wastewater. The anodic evolution of oxygen is a very common reaction in generic water treatments, and in particular in wastewater treatments when organic or biological substances have to be destroyed at extremely low levels. The effectiveness of nascent oxygen in the destruction of organic substances depends primarily on the potential of anodic evolution, which must be as high as possible, preferably without the need for the use of excessive current densities. Other industrial processes, for example in the field of organic electrosynthesis, can make use of the evolution of oxygen at high potential over the anode of the invention, however the oxidation of organic species in aqueous solutions undoubtedly represents its most diffuse use and greater Economic importance.

BACKGROUND OF THE INVENTION The anodes for evolution of oxygen at high overvoltage of the prior art are traditionally obtained on ceramic substrates, for example based on tin dioxide variously modified with other elements, mainly to impart a sufficient electrical conductivity; also the lead dioxide represents a material traditionally used for this purpose. The geometrical limitations of this type of substrates, however, have led to the development of high-oxygen overvoltage electrodes based on valve metals, which in their preferred configuration comprise a titanium or titanium alloy substrate, a ceramic protective intermediate layer, example based on titanium and tantalum oxides, and an external layer of low catalytic activity in which the tin dioxide represents once again the majority component, usually mixed with other elements such as copper, iridium and antimony; an electrode of this type, which also comprises an intermediate catalytic layer containing mainly tantalum and iridium oxides, is described in example 6 of WO 03/100135. Although the electrode of WO 03/100135 is able to offer interesting initial benefits in the indicated application, by evolving oxygen to potentials barely above 2 V with currents of 100 A / m2 in sulfuric solution, its duration is quite unsatisfactory. Indeed, although the anode mentioned above is provided with an external layer of low catalytic activity, in the common industrial running conditions the oxygen evolution potential tends to drop abruptly within a few hundred hours, together with the efficiency of elimination of organic species. Furthermore, from the description of WO 03/100135 it can be immediately noted how the preparation method of the relative electrode is quite complex for a large scale production, due to the fact that a large number of alternate layers of two different precursors (in the example, ten alternate layers of two hands each) should be applied. It is an object of the present invention to provide an anode for oxygen evolution operating at high overvoltage, indicatively greater than 2 V (NHE) at current densities not exceeding a few hundred A / m 2, which exceeds the limitations of the prior art presenting at the same time a longer duration in industrial driving conditions. It is a further object of the present invention to provide a method for the production of an anode for high overvoltage oxygen evolution characterized by easy industrial applicability. Under a first aspect, the invention consists of an anode obtained on a ceramic substrate or preferably on a substrate of titanium, titanium alloy or other valve metal substrate, comprising a first intermediate protective layer based on valve metal oxides as is notorious in the art, a second intermediate protective layer based on noble metals and an outer layer containing tin, copper and antimony oxides. In a preferred embodiment, the substrate of titanium or titanium alloy activated according to the invention is previously provided with an appropriate roughness profile, for example by means of a sandblasting treatment and subsequent etching in sulfuric acid. In another preferred embodiment, the first intermediate layer comprises a mixture of titanium and tantalum oxides; In another preferred embodiment, the second intermediate layer based on noble metals contains platinum, more preferably in an amount comprised between 10 and 24 g / m2. The outer layer contains oxides of tin, copper and antimony, optionally in combination with other elements. The tin content is preferably between 5 and 25 g / m2, that of antimony between 0.4 and 2 g / m2, and that of copper between 0.2 and 1 g / m2; in a still more preferred embodiment, the tin is present in an amount of at least 90% by weight of the total metal content. In another aspect, the invention consists of a method for the production of an anode for evolution of high-voltage oxygen, comprising the successive application of a first intermediate protective layer based on oxides of valve metals, of a second intermediate layer a noble metal base and an outer layer containing tin, copper and antimony oxides on a ceramic substrate or in valve metal. In a preferred embodiment, the substrate is titanium or titanium alloy, previously treated to impart a suitable roughness profile, for example by means of a sandblast followed by a pickling in sulfuric acid, as described in WO 03 / 076693. Other types of treatments are possible anyway, for example thermal spraying or plasma treatments or pickling with other corrosive media. In a preferred embodiment, the first intermediate layer is obtained by application of precursors, for example titanium and tantalum chlorides, and successive thermal decomposition, for example between 450 and 600 ° C; the application of the precursor can be effected, as is known in the art, by means of different individual or combined techniques, such as spraying or application with a brush or roller. In a preferred embodiment, the second intermediate layer is obtained by thermal decomposition of hexachloroplatinic acid at a temperature of 400-600 ° C, but other forms of application of noble metals, for example through a galvanic process, can also be practiced. Precursors of other noble metals can be included during the formation of the second intermediate layer, but the presence of platinum is particularly preferred. In a particularly preferred embodiment, the outer layer is applied using a single solution containing the precursors of the tin, copper and antimony oxides, for example the relative chlorides. The solution is applied according to the prior art and preferably decomposed between 450 and 600 ° C. The anode of the invention is suitable for evolving oxygen at high overvoltage, that is to a potential indicatively higher than 2 V (NHE) at current densities of a few hundred A / m 2, with much higher life times compared to those of the anode of WO 03/100135 or other anodes of the prior art. Without wishing to limit the present invention to any particular theory, it can be presumed that, in the case of WO 03/100135, the anode tends to form fractures or cracks in the coating, discovering some areas, however limited its extension, which have a high content of iridium or in any case a significantly lower oxygen overvoltage. In the case of the anode of the invention, the eventual formation of fractures or cracks would discover areas rich in platinum, over which the oxygen overvoltage is still quite high. An explanation of this type seems to be corroborated by the data reported in the attached figure.

BRIEF DESCRIPTION OF THE FIGURE Figure 1 shows some polarization curves relating to the evolution of oxygen on the anode of the invention.

DETAILED DESCRIPTION OF THE INVENTION In particular, the curves of Figure 1 refer to the evolution of oxygen in sodium sulfate at pH 5 and at 25 ° C. It is indicated by (1) the polarization curve relative to the anode according to the invention, with (2) that relative to the anode according to the invention, provided with the two intermediate layers, respectively based on platinum-based titanium and tantalum oxides , with (3) that relative to an anode only provided with the first intermediate layer based on titanium and tantalum oxides and an external layer based on iridium and tantalum oxides. In fact, curve (2) simulates the behavior of an anode according to the invention in which the outer layer based on tin, copper and antimony oxides is totally destroyed, while curve (3) simulates the destructive situation total of the outermost layer of the anode according to WO 03/100135. The invention will be further illustrated by the example below, which does not intend to limit its scope in any way, which is only defined by the appended claims. EXAMPLE A grade 1 titanium sheet according to ASTM B 265, dimensions 45 cm x 60 cm and 2 mm thick, was treated with corundum sandblasting and etching with 25% sulfuric acid containing 10 g / l of dissolved titanium, at a temperature of 87 ° C. A solution containing titanium and tantalum chlorides was applied to the sheet, at a concentration of Ti 0.11 M and Ta 0.03 M, by electrostatic spraying followed by roller application. Four coats of solution were applied until obtaining a total load of 0.87 g / m2 of deposit, drying between one hand and the successive one at 50 ° C for 10 minutes, and successively carrying out the thermal decomposition at 520 ° C for 15 minutes.

A first intermediate layer was obtained, on which was applied a second intermediate layer constituted by 20 g / m2 of Pt. The application was carried out in three hands, brushing hexachloroplatinic acid dispersed in eugenol and by thermal decomposition for 10 minutes at 500 ° C after each hand. The outer layer was finally applied from a solution of tin chlorides (IV) (94% by weight based on the total content of metals), copper (II) (2% by weight based on total metal content) and antimony ( 4% by weight based on the total metal content). The application was made by brushstroke in 16 hands, with drying cycles at 50 ° C and decomposition at 520 ° C after each hand. The electrode of the invention thus obtained was subjected to a polarization test under evolution of oxygen in sodium sulphate at pH 5 and 25 ° C, and the results are reported in figure 1 in the curve indicated as (1). In Figure 1 are also reported the polarization data obtained in the same conditions with an equivalent electrode devoid of external layer, and with an electrode provided with a first equivalent intermediate layer, and with an outer layer containing 24 g / m2 of oxides of tantalum (35% by weight) and iridium (65% by weight). These data are reported in the curves indicated respectively as (2) and (3). Finally, the electrode of the invention was subjected to an accelerated duration test in sulfuric acid at a concentration of 150 g / l at a temperature of 60 ° C, with a current density of 20 kA / m2. After 500 hours of accelerated testing, its oxygen evolution potential in sodium sulfate at pH 5 and 25 ° C was determined at the current density of 500 A / m2: the detected potential was found to be 2.15 V (NHE). An anode prepared according to WO 03/100135, subjected to the same test, has shown an oxygen evolution potential of 1.74 V (NHE) under the same conditions. As is apparent to a person skilled in the art, the invention can be practiced by providing other variations or modifications to the alleged examples. The above description will not be understood as limiting the invention, which can be practiced according to different embodiments without departing from its objectives, and whose scope is univocally defined by the appended claims. In the description and claims of the present application, the word "understand" and its variations such as "comprises" and "understood" are not intended to exclude the presence of other accessory elements or components.

Claims

CLAIMS 1. Anode for evolution of oxygen at high overvoltage, comprising a valve or ceramic metal substrate, a first intermediate layer based on valve metal oxides applied to said substrate, a second intermediate layer based on noble metals applied to said first intermediate layer, an outer layer that contains oxides of tin, copper and antimony. The anode of claim 1 wherein said valve metal substrate is made of titanium or titanium alloy. The anode according to claim 2 wherein said titanium or titanium alloy substrate has a roughness profile controlled by means of a treatment comprising an etching in sulfuric acid optionally prec by a sandblasting treatment. 4. The anode according to any one of the preceding claims wherein said first intermediate layer comprises titanium and tantalum oxides. 5. The anode according to any one of the preceding claims wherein said second intermediate layer comprises 10 to 24 g / m2 of platinum. The anode according to any one of the preceding claims wherein said outer layer comprises from 5 to 25 g / m2 of tin, from 0.4 to 2 g / m2 of antimony and from 0.2 to 1 g / m2 of copper. The anode according to claim 6 wherein the tin is present in said outer layer in an amount not less than 90% by weight of the total metal content. 8. Method for the production of an anode for evolution of oxygen at high overvoltage, which comprises applying a first intermediate layer based on oxides of valve metals to a metal or valve ceramic substrate, applying a second intermediate layer based on noble metals To said first intermediate layer, apply an outer layer containing tin, copper and antimony oxides. 9. The method according to claim 8 wherein said substrate is a substrate of titanium or titanium alloy with a controlled roughness profile, obtained by means of a sandblasting treatment and successive pickling in sulfuric acid. The method according to claim 8 or 9 wherein said first intermediate layer is applied by means of at least one method selected from spraying, brushing and roller treatment from a solution of titanium and tantalum chlorides, with successive thermal decomposition at a temperature between 450 and 600 ° C. 11. The method according to any one of claims 8 to 10 wherein said second intermediate layer is applied by thermal decomposition of a solution containing hexachloroplatinic acid at a temperature comprised between 400 and 600 ° C. The method according to any one of claims 8 to 11 wherein said outer layer is applied in a multiplicity of hands from a solution containing tin, antimony and copper chlorides, with successive thermal decomposition at a temperature comprised between 450 and 600 ° C. 13. Electrochemical process comprising the anodic evolution of oxygen at a potential greater than 2 V (NHE) on an electrode according to any one of claims 1 to 7. 14. The process according to claim 13 comprising the industrial treatment of water . 15. The process according to claim 14 wherein said treatment comprises the removal of organic molecules from wastewater.